This invention relates to liquid ring pumps, and more particularly to the shape of the port members of conically ported liquid ring pumps.
Liquid ring pumps are commercially made in two well known configurations. One of these configurations is commonly called a flat sided design (see, for example, Siemen U.S. Pat. No. 1,180,613). In flat sided pumps the ports which direct the gas to be compressed into and out of the rotor are formed in a flat plate which runs with close clearance to the axial end of the rotor. The direction of the fluid entering and exiting the rotor is axial, that is, parallel with the rotor shaft; hence flat sided pumps are also called axial flow ported pumps. The other configuration is commonly called a conical design. In this design (see, for example, Shearwood U.S. Pat. No. 3,712,764) the gas ports are formed in a conical structure which fits with close running clearance to a conical recess inside the end of the rotor. The fluid flow path exiting the rotor through the cone port is substantially radial; therefore, conical design pumps are also called radial flow ported pumps.
The conical structures of known designs are constructed with a small taper angle, typically around 8 degrees or less. A special case where the port structure is cylindrical is also produced.
This specification discloses a new design characterized by a porting structure which supports significant components of flow in both axial and radial directions. For the purpose of distinguishing it from the prior art it will be termed a mixed flow port structure in this disclosure. This development offers several improvements in cost and performance of liquid ring pumps, especially those of very wide construction, which will be described below. The significance of these improvements is best understood by first examining the advantages and disadvantages of the prior construction methods.
The two known design configurations have distinct advantages and disadvantages associated largely with the porting configuration and the design constraints associated with either case. For instance, an axial flow or flat sided design has the following advantages over the radial flow conical design.
A flat sided port plate is potentially a simpler structure to manufacture than a radial flow cone. For instance, it can be fabricated from steel plate and ground flat through relatively economical machining processes. A cone is usually formed by a casting process and machined by a turning process which in some cases may be more expensive.
The flat sided head may be cast more easily since it is fully open on the side covered by the port plate. A radial flow conical head design is not as open, which complicates the support of coring used in the casting process.
The load on the shaft of a flat sided pump is distributed closer to the bearings, which may result in a smaller diameter shaft for an equivalent load. Also, the radial clearance between the rotor and stationary parts is not as critical as with radial flow conical pumps; therefore the shaft stiffness is less critical.
The rotor machining process for flat sided rotors does not include an operation for the radial flow cone recess.
The rotor blades of axial flow pumps are supported (reinforced) along the full length of the rotor hub, thereby minimizing any localized high stress areas. The blades in radial flow designs are not well supported in the area where the port is inserted, which may lead to areas of stress concentration.
Some of the disadvantages of the flat sided design relative to radial flow conical pumps are as follows.
The axial flow design may not be as efficient as radial flow conical pumps because the port velocities may be higher and cause higher entry and exit pressure losses. This becomes increasingly significant as the pump width relative to diameter increases. The port sizes of axial flow pumps are relatively fixed, independent of pump width. Radial flow ported pumps offer more dimensional control of the port velocities by varying the base diameter and/or length of insertion of the cone into the rotor.
In addition, the conical port structure offers a plenum under the rotor which better distributes the flow into and out of the rotor.
The axial direction of flat sided discharge flow limits the water handling ability of flat sided pumps. This disadvantage is explained as follows. The flow discharged from a liquid ring pump is inherently two-phase in nature--liquid and gas. A characteristic of two-phase flow is that the liquid component will not change direction unless acted upon by an external influence, for instance, by a guide vane. Since the flow direction within the rotor (relative to the rotor) is primarily radial, and there is no external influence other than the radial blades, excess liquid is more prone to stay within the rotor than to be discharged. This contrasts to a radial flow conical design in which the direction of liquid flow relative to the rotor is the same as the direction of discharge. Therefore, excess liquid in a radial flow conical design is readily discharged.
The consequence is that flat sided design performance is more adversely affected by liquid in the incoming gas stream than a radial flow conical pump. In the extreme this results in an earlier onset of cavitation and/or rotor breakage. Also, as with the port velocities, the problem associated with excess liquid increases as the pump width relative to its diameter increases. An axial flow port becomes more remote from the source of the problem as pump width increases, and this compounds the problem of purging excess liquid.
Flat sided pumps have reduced condensing ability relative to radial flow conical pump designs. Because of the higher inlet port velocities, the effect of introducing liquid spray into the inlet gas stream causes higher pressure drops in flat sided pumps than in conical pumps. Therefore the significant advantage of condensing the vapor content of inlet gas streams is reduced in flat sided designs. This problem is amplified by the inability of flat sided designs to safely handle as much liquid as a fraction of the gas/vapor volume, since condensing ability is directly proportional to the liquid fraction.
The performance of flat sided pumps is very sensitive to the axial clearance between the rotor and port plate. Hence it is often not practical to control flat sided clearances by the use of shims. This leads to a greater variation of performance of production lots of pumps. In a radial flow conical design constructed with, for instance, an angle of 8 degrees, there is a 7 to 1 amplification of the clearance setting. Therefore critical clearances between the rotor and cone can be controlled precisely with shim adjustments of the axial position of the parts and more uniformity in performance can be achieved.
As is evident from the above discussion, several of the advantages of flat sided pumps may lead to a lower manufacturing cost relative to conical pumps of the same displacement. However, the lower manufacturing cost comes at a sacrifice in performance, liquid handling, and condensing ability. These are attributes which contribute markedly to the reliability and marketability of the products. Also, it is apparent from the above discussion that the poor attributes of the flat sided design worsen as the relative width increases.
As is known by pump designers, a key to improving the cost of liquid ring designs is to extend the relative width. The reason for this can be explained by examination of the interaction between part diameter and part length on the cost of manufacturing processes. Experience shows that if the diameter is held constant, the cost of a pump divided by its displacement (expressed as dollars per cubic feet per minute or $/CFM) usually decreases as the width increases until a minimum point is reached; beyond that point the cost per displacement increases. The minimum point is determined by both mechanical and performance limits. For example, one of several factors is that the shaft diameter becomes so large that shaft cost becomes disproportionate and the size of the shaft takes away a disproportionate share of the bucket volume (volume between adjacent rotor blades), increasing dollars and dropping CFM.
Generally speaking, for prior art double ended pump designs (e.g., as in the above--mentioned Shearwood patent), the minimum $/CFM occurs at a cumulative axial rotor blade length (excluding the thickness of the end and center shrouds) of about 1.3 times the rotor diameter. A benefit of the mixed flow cone development is an extension of the minimum cost limit to axial rotor blade lengths beyond 1.3 times the rotor diameter, as will be described in detail below.
Jennings U.S. Pat. No. 1,718,294 shows conically ported liquid ring pumps with relatively large cone angles (approximately 18 degrees in FIG. 1 and approximately 12 degrees in FIG. 4). However, Jennings shows the rotor shrouded immediately adjacent to the ports in the cones and in such a way as to substantially preclude any axial component of fluid flow between the cones and the rotor.
In view of the foregoing, it is an object of this invention to provide improved liquid ring pumps.
It is a more particular object of this invention to provide liquid ring pumps which combine some of the benefits of both axial flow and radial flow pump designs.
It is still another object of this invention to provide liquid ring pumps having many of the advantages of radial flow design pumps, but which can be economically constructed with greater axial rotor blade length to rotor diameter ratios than are generally economical for known radial flow pumps.